A common type of air filter for internal combustion engines has a cylindrical filter housing with the outlet opening at one end. The pipe stub for connecting the filter to an intake conduit is fixed to the housing end plate and the tubular filter element has one end against the end surface surrounding the outlet with a seal therebetween, which is compressed when the filter body is pressed against the outlet end plate by the other end plate of the housing which serves as a cover when in place.
These axially sealing filters have a major disadvantage. Due to the fact that the filter must be fitted between the housing end plates, special attention must be payed to the axial dimensions of the filter element and the filter housing. Any deviations can result in leakage or damage to the filter element.
A known solution to this problem is to seal the filter element radially. The filter element is provided on its inner lateral surface at its outlet end with an inner sealing ring, which seals against a pipe end forming an extension of the outlet stub extending into the filter housing. The axial fit between the filter element and the filter housing is thereby not as critical as in axially sealing filters.
Common to the two known filter types described is, however, that it is difficult when replacing a filter to prevent dirt on the surface of the used filter or on the inside of the filter housing from getting onto the "clean side" of the new filter.
For axially sealing filters it is known to use a secondary or safety filter (which will be referred to below as a secondary filter) concentrically inside the tubular main filter. Such a filter prevents particles from getting on to the clean side of the air filter when replacing the primary filter. It also prevents particles, which for some reason have passed by the primary filter, if it is damaged, for example, from being sucked into the engine. In order to fix the secondary filter in the filter housing, according to one known design, a so-called center rod is used which extends centrally through the filter and presses one end of the filter against a housing end plate. Such an arrangement places great demands on the rigidity of the housing and cannot be used in an entirely plastic housing.